Chapter 8: Biology

Listening to Electric Fish

Listening to the Elephant Nose fish

At your local tropical fish store, you can find a particularly
wonderful creature called an Elephant Nose fish, or more properly,
Gnathonemus petersii, a member of the Mormyrid family.
[Click on photo for larger picture]
Besides the obvious visual interest of the animal, there is another
fascinating aspect to it -- it emits pulses of electricity into the
water, with which it locates food, other fish, and potential mates.
It is easy to listen in on these electrical signals with simple
inexpensive equipment, such as a piezoelectric earphone, or a
small amplifier.
A piezoelectric earphone is a simple device I have described
earlier (in the
crystal radio section. You can get them at Radio Shack
in their crystal radio kits, or you can get them from
our
catalog. They are very sensitive to small
electrical signals, and so they are perfect for detecting the
electrical signals from the Elephant Nose fish, and converting
them into sound you can hear.
A piezoelectric earphone[Click on photo for larger picture]
To listen in on the fish, simply put one wire from the earphone into the
water at one side of the fishtank, and the other wire into the
water at the other side of the fishtank, and put the earphone up
to your ear.
If you click
here you can hear what it sounds like.
If the fish is sitting still, the electrical pulses will be
infrequent. But as the fish moves around, it increases the
frequency of the pulses, until it is almost a buzz. It does
this because as it moves faster, it needs more information
about its surroundings in order to navigate. It uses the pulses
like radar, to avoid obstacles, to find food, to avoid predators,
and to locate other members of its species.
To enable an entire room to hear the electric fish,
you need an amplifier. A stero system or boombox
usually has an auxilliary input or phonograph input
that you can use. Simply plug in a cable, and place
the two wires in the tank like we did for the earphone.
In the photo below, I used a Radio Shack battery powered
amplifier, which is small and easy to carry to a fish store.
The amplifier is the Radio Shack part number 277-1008.
[Click on photo for larger picture]
A plug goes into the input jack, and two alligator clips
connect the wires from the plug to two bare copper wires
running into the water on the left and right sides of the
tank. The fish has been temprarily placed in a tiny tank
to make photographing it easier. This tank is much too
small to keep the fish in for more than about an hour.
With the amplifier, an entire room can easily hear the clicks
and buzzes of the fish as it reacts to the environment.

The Nature of the Signal

With an oscilloscope or a computer sound card, we can capture
the signal and look at a picture of it. The graph below is
a snapshot of a bit of fast burst from the fish as it darted
around the tank.
[Click on graph for larger picture]
A closeup view of a single pulse is shown below.
[Click on graph for larger picture]
By sinply putting a microphone up to the amplifier you
can capture the sounds to a computer sound card and
examine the graphs in detail. The graphs on this page
were captured with an expensive oscilloscope, and then
graphed with a spreadsheet program on a computer, but
you don't need expensive equipment to get similar
results. A sound card is more than enough to get all
the details at these frequencies.

The Electric Organ Discharge

The electric organ of the Elephant Nose fish is
clearly visible. It is the pair of white bands
running top to bottom between the top and bottom
fins that form a skirt in the middle of the body.
[Click on photo for larger picture]
This is the organ that generates the signal. It is
a modified muscle. To receive electric signals, either
from other fish or from echoes of its own discharge,
the Elephant Nose fish has three different types of
receptors. These are the mormyromasts, the
knollenorgans, and the ampullary receptors.
The mormyromasts are the organs that detect the echoes
of the electric organ discharge. They are what enable
the fish to navigate in murky water and find prey.
The knollenorgans detect the electric organ discharge
of other elephant nose fish, and thus aid in communication
and finding mates.
The ampullary receptors measure the low frequency electric
fields emitted by other aquatic animals. Similar ampullary
receptors are used by catfish and sharks.
Fish are very sensitive to tiny amounts of polutants in their
environment. In some towns in Germany, the elephant nose
fish is used to detect very small amounts of lead and trichloroethylene
in the city's water supply. Because the electric discharges
are so easy to detect and monitor with a computer, this method
is cheaper than chemical tests, and can be done continuously.
The number of discharges per minute drops off measurably when
the levels of either pollutant rises, even at levels far below those
considered dangerous.

Other Electric Fish

Some of the better known electric fish are the South American
electric eel Electrophorus electricus,
and the African electric catfish Malopterurus electricus.
Another famous electric fish is the Mediteranean electric ray
Torpedo torpedo. There is also an electric catfish from
China, the Parasilurus asota.
These are large fish with powerful electric discharge organs.
They use the electric discharge to stun their prey and deter predators.
Other electric fish are more like the elephant nose fish,
where the electric discharge is very small, and used for navigation and
communication.
In the same family of fish (called mormyriform fish) as the
elephant nose are several other species, such as Pollimyrus isidori,
Gymnarchus niloticus, and Brienomyrus brachyistius.
Another family of weakly electric fish are the South American
gymnotoid fish, such as Hypopomus artedi,
Sternopygus, and Eigenmannia. While Hypopomus
produces pulses like the elephant nose fish, the others produce
pure continuous sine waves.
The South American gymnotoid fish Eigenmannia virescens
(the Glass Knifefish), (the Brown
Ghost Knife fish), and Apteronotus albifrons (the Black Ghost
Knife fish) are easy to find in tropical fish stores. They are
all of the continuous sine wave type, rather than the pulse type.
For more information on electric fish, enter any of the scientific
names mentioned above into an Internet search engine. There is an
amazing amount of material available on these animals on the Web.